A character with a big mouth says something stupid or, better yet, self-incriminating. Usually the twin trope to or immediately followed by Did I Just Say That Out Loud?.
The trope name comes from the phrase "to put your foot in your mouth" (also playfully referred to as "foot-in-mouth disease", a pun on the very serious livestock illness "foot-and-mouth disease").note Which in turn is not to be confused with the rather less serious hand, foot, and mouth disease, which primarily afflicts small children and is generally speaking a week's nuisance at worst.
Closely related to Ignore The Disability.
Can lead to either Change the Uncomfortable Subject or Digging Yourself Deeper. Sometimes both.
Elecktrum's Narnia Fics are full of this trope. One fic revolves around it completely, and another has a chapter revolving around the similar phrase "eating crow." Her OCs and minor characters also get into the act, especially where Edmund is concerned:
Tumnus unintentionally insults Edmund by implying that Edmund had worked for the white witch in "The Conscience of the King".
In "Black Dwarfs, Blue River" an overzealous faun angrily tells Edmund regarding the persecution of families of some satyrs who fought on the witch's side that the wives and children (who did not fight for the witch) were "Sons of traitors [who] deserve what what they get." Edmund responds cooly: "so what will you call my sons?" Oops.
In the prequel to Respawn Of The Dead, Heavy accidentally insults Medic a few times. But, to be fair, it's pretty clear that Medic is only insulted by being told he sounds like Greta Garbo because he has no idea he was practically quoting her.
In the Back to the Future fanfic Homecoming, George, not knowing about the age gap between Doc and Clara, asks where "Mrs. Brown" is. He's mortified when he finds out that Clara is Doc's wife, not his stepdaughter, and apologizes profusely.
In Kira Sweetheart Light has one of these moments when he discovers that L has been abused in the past.
Invoked by Buffy in "Dopplegangland" when she tries to apologize to Willow (not realizing she is talking to Willow's alternate universe vampire double) for an unfortunate comment earlier in the episode, saying, "You know how my foot likes to live in my mouth."
Referenced fairly often on Mash. In one case, BJ claims Charles has "put a whole shoe store" in his mouth." (That example, interestingly, was incorrect, as Charles had fully meant to offend everyone.)
In Firefly, Simon does it virtually every time he tries to have a conversation with Kaylee.
Jeff: Oh, yeah! Especially the tongues part. I love getting all that extra tongue. You know, sometimes I eat really cold ice cream just so that my tongue goes numb and it feels like someone else's... But then we all get lonely sometimes.
Star Trek: The Next Generation: During the 3rd season episode, Hollow Pursuits, Capt. Picard accidentally calls Barclay by his unofficial nickname Broccoli. Data tries to put a positive spin on the situation by referencing psychology but really only makes the situation worse.
In one episode of The Golden Girls, Blanche had an epic one. She meets and asks out a man she met in a bar, not realizing at the time (since he was sitting down) that he was blind. When he comes to pick her up for their date, she enters the room, saying things like, "Aren't I the prettiest thing you've ever seen?" and "What's the matter, are you blinded by my beauty?" She finally takes a good look, sees his cane, and stops in her tracks. Sophia directly addresses it, saying, "Want a glass of water to wash down your foot?"
JAG: Lt. Bud Roberts falls victim to this fairly often, especially in the pilot movie whenever dealing with Lt. Caitlin Pike. His case of Foot-in-Mouth Disease is particularly unfortunate given that his original job is Public Affairs.
NCIS: Jimmy Palmer is very prone to this. He does not seem to have heard the old adage that "a closed mouth gathers no feet". His foot tasting episodes routinely escalate to Digging Yourself Deeper territory.
In Doctor Who, the Tenth Doctor has a bad habit of doing this while talking to Queen Elizabeth I, repeatedly saying terribly offensive or regrettable things under the assumption she was a zygon duplicate. It gets them married.
A Moody Christmas. In the first episode Dan mistakes Cora for a door-knocking Red Cross volunteer because of her shirt, and donates some money. It turns out she is his cousin's new girlfriend, isn't a volunteer, and designed and hand-made the shirt herself.
In this strip Pirta and Swampy get into a brief discussion of his nickname. After Swampy asks about her name in an idiot moment for him, he comments that he can skip the appetizer, as he'll just chew on his own foot.
In In this strip, a customer that's been hassling Pirta for a date is asked if he'd be okay with the situation were their positions reversed, and she were trying to get him to cheat on his girlfriend. He accidentally lets slip that he'd be fine with cheating on his girlfriend Jenny for a date with Pirta. He winds up thrown bodily out of the store after being beat up by Pirta for his troubles.
Aaryanna has the bad habit of invading the dreams her boyfriend Merlitz, but when she tried to convince Dan to embrace his Cubi's cultural heritage, she commented that's nothing better for a cubi than simply invade other peoples' dreams... and she said that within Merlitz's earshot.
Simmons:(referring to Grey's rescue of Alex and Rick) C'mon, I saw that one of the officers was female, and a cute one at that—I know you, Grey. Grey: Her?! Please, she's way too uptight. I don't have time for girls like that. Simmons: What, you think you even stand a chance? Grey: Give me a break. She may have an attitude, but if I wanted her, I could have her by the end of the night. Alex: Hm, I'll take that bet.
A Running Gag in Schlock Mercenary is the multiple creative variations used to call someone (usually Tagon, but he was by no means the only guilty party) out when they said something particularly embarrassing to or about another character while in earshot. It doesn't get mentioned as frequently in more recent strips as it did early on, but still pops up from time to time.